To provide a two-stage bolt and fastening structure using the two-stage bolt easy to handle and prevented from getting loose inadvertently without deforming a fastened object or being deformed itself when fastened.

An inner bolt member 30 having an external thread part 31 at one end is accommodated in a cylindrical outer bolt member 20 having an external thread part 21 at one end. An engaging internal thread part 25 thread- fastened to the external thread part 31 is formed at a part of the inner surface of the outer bolt member 20. A diameter enlarged head 22 is formed at the other end of the outer bolt member 20, and a head 32 formed at the other end of the inner bolt member 30 is engaged with the head 22. A main thread part and an auxiliary thread part are formed at a fastened member. The external thread part 21 is thread-fastened to the main thread part, and the external thread part 31 is thread-fastened to the auxiliary thread part. Unless the external thread part 31 is unfastened from the auxiliary thread part, the external thread part 21 does not get loose relative to the main thread part.